Chemically temperable glass sheet

ABSTRACT

The invention relates to a glass sheet having a boron-and lithium-free glass composition comprising the following in weight percentage, expressed with respect to the total weight of glass : 65≦SiO 2 ≦78% 5≦Na 2 O≦20% 0≦K 2 O&lt;5% 1≦Al 2 O 3  ≦4% 0≦CaO&lt;4.5% 4≦MgO≦12%; as well as a (CaO/MgO) ratio which is less than 1. The invention corresponds to an easy chemically-temperable soda-silica type glass composition, which is more suited for mass production than aluminosilicate glass, and therefore is available at low cost, and with a base glass/matrix composition that is close to or very similar to compositions already used in existing mass production.

FIELD OF THE INVENTION

The present invention relates to a glass sheet which is able to bechemically tempered/strengthened. In particular, the present inventionrelates to a glass sheet which is able to be easily chemicallytempered/strengthened and which is inexpensive and easy to produce.

Chemically strengthened glass sheets are finding increasing applicationsin specialized glazing jobs where a mechanical resistance isrequired/mandatory, in a monolithic or laminated form, liketransportation (i.e. aeronautical, automotive), building/architectureand display industries. Amongst such applications, the display industryhas become in the several past years a huge market on demand forchemically strengthened transparent glass sheets as protective/coverglass, viewing window or (touch)screen for numerous electronic deviceslike mobile phones, smartphones, TV, computers, digital cameras, etc.Indeed, as many of these devices are portable, the glass used ismechanically solicited a lot and it is therefore highly desirable thatit is able to tolerate impact and/or damage, such as scratches orimpact, during use and transport. Chemical strengthening is even more ofgreat importance in the domain of displays because such a domainrequires glass sheets of low thickness (as low as less than 1 mm) andbecause chemical strengthening is known as the process of choice tomechanically reinforce (ultra-)thin glass sheets. For weight reasons, itis also advantageous to use thin glass sheets as cover glass for solar,thermal or photovoltaic devices.

SOLUTIONS OF THE PRIOR ART

The chemical strengthening of a glass article is a heat inducedion-exchange, involving replacement of smaller alkali sodium ions in thesurface layer of glass by larger ions, for example alkali potassiumions. Increased surface compression stress occurs in the glass as thelarger ions “wedge” into the small sites formerly occupied by the sodiumions. Such a chemical treatment is generally carried out by immergingthe glass in an ion-exchange molten bath containing one or more moltensalt(s) of the larger ions, with a precise control of temperature andtime. The rupture strength of a glass article which has been so treatedis thus increased by a value approximately equal to the surfacecompressive stress generated.

Nevertheless, a damage capable of affecting the surface of a chemicallystrengthened glass during its use leads to a decrease in thisstrengthening effect and can even annihilate it if the damage is suchthat the layer under compression is penetrated. In consequence,depending on the use intended for the chemically strengthened glass,focus is made on achieving a high value of surface compressive stress(or “CS”) and/or a high value of thickness of the layer undercompression (which is associated with the parameter called the “depth oflayer” or “DoL”, namely the depth reached by the ions introduced) whichideally at least equal to the depth of the largest possibledefect/damage that the glass may undergo. The combination of these twoparameters are generally considered to define appropriately the qualityof the resulting mechanical strength.

In particular, in the display domain, when using a “piece-by-pieceprocess” to produce chemically strengthened glass sheets (cutting tofinal size is carried out before tempering treatment), a high value ofDoL (preferably higher than 10 microns and very preferably higher than12 microns or even better higher than 15 microns) is searched for edgestrength, while when using a “sheet process” (cutting to final size iscarried out after tempering treatment), “central tension” (defined as(CS*DoL)/(glass thickness−2*DoL)) must be kept low.

It is also known that the two strengthening parameters also dependsignificantly, for a given glass composition, on the conditions oftemperature and time of the ion exchange process. Thus, the thickness ofthe layer under compression increases with the temperature and with theduration of the ion-exchange according to the known diffusion laws. Butthe higher the temperature, the more rapidly the stresses induced by theion exchange relax. Likewise, extending the treatment for a too longperiod allows giving the stresses the necessary time to relax and thusresults in a less degree of toughening. The conditions to be chosen forthe process therefore reside generally in a compromise between theoptimum temperature and the minimum duration, to optimize process cost.

To lower the cost of the chemical strengthening (limiting durationand/or temperature to reach searched values of compressive stress andDOL), a lot of glass compositions which are “easy chemically temperable”(meaning that they especially favour ion exchange) have been proposed(merely described or already on the market) but they generally havevarious drawbacks.

Many of them comprise ingredients originating from expensive rawmaterials and/or considerably modifying the physical properties of theglass (molten or final). Some of the chemically temperable glasscompositions known contain, for example, significant contents of lithiumand/or boron. However, lithium has the disadvantage of increasing thedensity of the glass while boron has the disadvantage to cause sometimesformation of ream by its evaporation and furnace wall/refractoriescorrosion. Moreover, both have the additional and significant drawbackto greatly increase final glass price, due to high price of theircorresponding raw materials.

Aluminosilicate-type glass compositions, such as for example thosedescribed in US Patent Application US2012/0196110 A1, the GORILLA® glassproduct from Corning or the DragonTrail® glass product from Asahi GlassCo., are also known to be very efficient for chemical tempering.However, they have a lot of drawbacks. Their high temperature propertiesmake them very difficult to produce (viscosity, fining ability, forming,refractories corrosion). Their cost is relatively high due toexpensiveness of some raw materials to use (i.e. alumina) and due to thehigh temperatures required for their production (high content ofenergy/fuel).

Contrary to aluminosilicate glass compositions, soda-lime-silica glasscompositions are generally not considered as good candidates for easychemically temperable compositions, even if they are by far lessexpensive.

Finally, it is known that it is quite difficult to modify, evenslightly, a glass composition, because:

-   -   a glass production line, and in particular a float line,        represents considerable investment and it is not repairable if        the composition causes, for example, damages to the        refractories; and    -   the transition time while changing from a composition to another        is one parameter which is of high importance when producing        glass, because if long, the production cost of the final glass        is drastically negatively impacted.

Accordingly, there is a demand of the market in the display domain inparticular for an enable chemically-temperable soda-lime-silica-typeglass composition, which is more suited for mass production thanaluminosilicate glass, and therefore is available at low cost, and witha base glass /matrix composition that is close to or very similar tocompositions already used in existing mass production.

OBJECTIVES OF THE INVENTION

The objective of the invention in particular is to remedy the citeddisadvantages and resolving the technical problem, i.e. to provide aglass composition which is easy chemically temperable or, in otherwords, more favourable to ion exchange than conventionalsoda-lime-silica glass compositions.

Another objective of the invention in at least one of its embodiments isto provide a glass composition which is easy chemically temperable andwhich allows reaching strengthening parameters appropriate for a“piece-by-piece” process used to produce cover glass for display devices(edge strength obtained typically by DoL>10-15 microns). In particular,an objective of the invention in such a context is to provide a glasscomposition which is easy chemically temperable and which allowsobtaining great exchange depth, while keeping compressive stress valuesthat result in a better reinforcement of glass.

Another objective of the invention in at least one of its embodiments isto provide a glass composition which is easy chemically temperable andeasy to produce, in particular on an existing line of production ofclassical soda-lime-silica glass. In particular, an objective of theinvention in such a context is to provide a glass composition which iseasy chemically temperable and which does not require long transitiontime when passing from the production of the classical soda-lime-silicacomposition to the temperable composition (and vice-versa). Still insuch a context, an objective of the invention in such a context is toprovide a glass composition which is easy chemically temperable andwhich does not require to use raw materials, techniques and/orindustrial installations which are different from those employed forclassical soda-lime-silica glass ordinary produced (or, in other words,compatible with classical float process). More particularly, anobjective of the invention in at least one of its embodiments is toprovide a glass composition which is easy chemically temperable and withtargeted properties (lower viscosity, lower working point temperature,melting point<1550-1500° C., sulfates fining ability, low refractoriescorrosion, appropriate devitrification temperature), thereby avoidingknown drawbacks of alumino-silicate composition and making compositioncompatible with existing tools for production of soda-lime glass.

Finally, another objective of the invention is to provide a solution tothe disadvantages to the prior art that is simple, quick and, above all,economical.

OUTLINE OF THE INVENTION

The invention relates to a glass sheet having a boron- and lithium-freeglass composition comprising the following in weight percentage,expressed with respect to the total weight of glass:

-   -   65≦SiO₂≦78%    -   5≦Na₂O≦20%    -   0≦K₂O<5%    -   1≦Al₂O₃<4%    -   0≦CaO<4.5%    -   4≦MgO≦12%;    -   as well as a (CaO/MgO) ratio which is less than 1.

Hence, the invention rests on a novel and inventive approach, since itenables a solution to be found for the disadvantages of prior art, inparticular the disadvantages of aluminosilicate glasses while keeping,at least partially, their advantages.

The inventors have indeed found that it is possible to obtain an easychemically temperable glass sheet which is unexpensive and easy to massproduce by combining in a soda-silica glass matrix, a very low aluminaand CaO content and a “reverse” CaO/MgO ratio in comparison withclassical industrial glass compositions (including soda-lime-silica andaluminosilicate glasses, with typical values for that ratio above 1).

Throughout the present text, when a range is indicated, the extremitiesare included. In addition, all the integral and subdomain values in thenumerical range are expressly included as if explicitly written. Alsothroughout the present text, the values of content as percentages arevalues by weight (also mentioned as wt %), expressed with respect to thetotal weight of the glass.

Other features and advantages of the invention will be made clearer fromreading the following description of preferred embodiments given by wayof simple illustrative and non-restrictive examples.

The glass sheet of the invention is made of a soda-silica glasscomposition/matrix, comprising SiO₂ and Na₂O as the main components andfurther comprising MgO, Al₂O₃, etc and optionally CaO, K₂O etc.

The glass sheet of the invention is able to be chemically tempered or,in other words, ion-exchangeable/able to undergo an ion-exchange.

The glass sheet of the invention may be a glass sheet obtained by thefloat process, a drawing process, a rolling process or any other processknown to manufacture a glass sheet starting from a molten glasscomposition. According to a preferred embodiment, the glass sheet is afloat glass sheet. The term “float glass sheet” is understood to mean aglass sheet formed by the float process, which consists in pouring themolten glass onto a bath of molten tin, under reducing conditions. Afloat glass sheet comprises, in a known way, a “tin face”, that is tosay a face enriched in tin in the body of the glass close to the surfaceof the sheet. The term “enrichment in tin” is understood to mean anincrease in the concentration of tin with respect to the composition ofthe glass at the core, which may or may not be substantially zero(devoid of tin). Therefore, a float glass sheet can be easilydistinguished from sheets obtained by other glassmaking processes, inparticular by the tin oxide content which may be measured, for example,by electronic microprobe to a depth of ˜10 microns. In many cases and asillustration, this content lies between 1 and 5 wt %, integrated overthe first 10 microns starting from the surface.

The glass sheet according to the invention may have varied andrelatively large sizes. It can, for example, have sizes ranging up to3.21 m×6 m or 3.21 m×5.50 m or 3.21 m×5.10 m or 3.21 m×4.50 m (“PLF”glass sheet) or also, for example, 3.21 m×2.55 m or 3.21 m×2.25 m (“DLF”glass sheet).

The glass sheet according to the invention may have a thickness of from0.1 to 25 mm. Advantageously, in the case of display applications, theglass sheet according to the invention has preferably a thickness offrom 0.1 to 6 mm. More preferably, in the case of display applicationsand for reasons of weight, the thickness of the glass sheet according tothe invention is of from 0.1 to 2.2 mm.

According to the invention, the composition of the glass sheet isboron-free. This meant that boron is not intentionally added in theglass batch/raw materials and that, if it is present, B₂O₃ content inthe composition of the glass sheet reaches only level of an impurityunavoidably included in the production. For example, B₂O₃ content in thecomposition of the glass sheet of the invention is less than 0.01 oreven better less than 0.005 wt %.

According to the invention, the composition of the glass sheet islithium-free. This meant that lithium is not intentionally added in theglass batch/raw materials and that, if it is present, Li₂O content inthe composition of the glass sheet reaches only level of an impurityunavoidably included in the production. For example, Li₂O content in thecomposition of the glass sheet of the invention is less than 0.01 wt %or even better less than 0.005 wt %.

According to the invention, the composition of the glass sheet comprises: 1≦Al₂O₃<4 wt %. Preferably, the composition of the glass sheetcomprises : 1≦Al₂O₃≦3 wt %. Alternatively, the composition of the glasssheet comprises : 2≦Al₂O₃<4 wt %. More preferably, the composition ofthe glass sheet comprises : 2≦Al₂O₃≦3 wt %.

According to the invention, the composition of the glass sheet comprises: 0≦CaO<4.5 wt %. Preferably, the composition of the glass sheetcomprises : 0≦CaO<4 wt % and more preferably, 0≦CaO<3.5 wt %. In a veryparticularly preferred embodiment, the composition of the glass sheetcomprises: 0≦CaO<3 wt %. In the most preferred embodiment, thecomposition of the glass sheet comprises: 0≦CaO<2 wt %.

According to the invention, the composition of the glass sheetcomprises: 4≦MgO≦12 wt %. Preferably, the composition of the glass sheetcomprises: 5.5≦MgO≦10 wt % and more preferably, 6≦MgO≦10 wt %.

According to the invention, the composition of the glass sheetcomprises: 0≦K₂O<5 wt %. Preferably, the composition of the glass sheetcomprises: 0≦K₂O<4 wt % and more preferably, 0≦K₂O<3 wt %, even better0≦K₂O<2 wt %.

According to the invention, the composition of the glass sheet comprisesa (CaO/MgO) ratio which is less than 1. Preferably, the composition ofthe glass sheet comprises a (CaO/MgO) ratio which is less than or equalto 0.45. More preferably, the composition of the glass sheet comprises a(CaO/MgO) ratio which is less than or equal to 0.4. In a veryparticularly preferred embodiment, the composition of the glass sheetcomprises a (CaO/MgO) ratio which is less than or equal to 0.2. In amost preferred embodiment of the invention, the composition comprises a(CaO/MgO) ratio which is less than or equal to 0.1.

According to a first embodiment of the invention, the compositioncomprises total iron (expressed in the form of Fe₂O₃) in a contentranging from 0.002 to 1.7% by weight. Preferably, the composition of theinvention comprises a total iron (expressed in terms of Fe₂O₃) contentranging from 0.002 to 0.6 wt % and, more preferably, ranging from 0.002to 0.2 wt %.

Preferably, the composition comprises total iron as follows:0.06<Fe₂O₃≦1.7% by weight. More preferably, the composition comprisestotal iron as follows: 0.06<Fe₂O₃≦0.6% by weight, and the mostpreferably, 0.06<Fe₂O₃≦0.2% by weight.

A total iron (expressed in the form of Fe₂O₃) content of less than orequal to 0.2 wt % makes it possible to obtain a glass sheet with limitedvisible coloration. The minimum value of 0.002 wt %, and preferably 0.06wt %, makes it possible not to be excessively damaging to the cost ofthe glass as such, low iron values often require expensive, very pure,starting materials and also purification of these. The minimum value of0.06 wt % of total iron allows also to promote molten glass convectionin a furnace and to yield better glass uniformity.

According to a particularly preferred embodiment, the composition of theglass sheet of the invention comprises the following in weightpercentage, expressed with respect to the total weight of glass:

-   -   65≦SiO₂≦78%    -   10≦Na₂O≦20%    -   0≦K₂O<4%    -   2≦Al₂O₃<3%    -   0≦CaO<3.5%    -   4≦MgO≦12%;    -   total iron: 0.06<Fe₂O₃≦0.2 wt %; and    -   a (CaO/MgO) ratio which is less than 0.45.

According to this last embodiment, the composition of the glass sheet ofthe invention more preferably comprises:

-   -   65≦SiO₂≦78%    -   10≦Na₂O≦20%    -   0≦K₂O<3%    -   2≦Al₂O₃<3%    -   0≦CaO<3.5%    -   6≦MgO≦10%;    -   total iron: 0.06<Fe₂O₃≦0.2 wt %; and    -   a (CaO/MgO) ratio which is less than 0.45.

According to this last embodiment and most preferably, the compositionof the glass sheet of the invention comprises:

-   -   65≦SiO₂≦78%    -   10≦Na₂O≦20%    -   0≦K₂O<3%    -   2≦Al₂O₃<3%    -   0≦CaO<2%    -   6≦MgO≦10%;    -   total iron: 0.06<Fe₂O₃≦0.2 wt %; and    -   a (CaO/MgO) ratio which is less than 0.2.

According to another embodiment, the composition of the glass sheetcomprises ZnO in an content lower than 0.1 wt % Preferably, thecomposition of the glass sheet is free of ZnO. This meant that theelement zinc is not intentionally added in the glass batch/raw materialsand that, if it is present, ZnO content in the composition of the glasssheet reaches only level of an impurity unavoidably included in theproduction.

According to another embodiment, the composition of the glass sheetcomprises ZrO₂ in an content lower than 0.1 wt %. Preferably, thecomposition of the glass sheet is free of ZrO₂. This meant that theelement zirconium is not intentionally added in the glass batch/rawmaterials and that, if it is present, ZrO₂ content in the composition ofthe glass sheet reaches only level of an impurity unavoidably includedin the production.

According to still another embodiment, the composition of the glasssheet comprises BaO in an content lower than 0.1 wt %. Preferably, thecomposition of the glass sheet is free of BaO. This meant that theelement zinc is not intentionally added in the glass batch/raw materialsand that, if it is present, BaO content in the composition of the glasssheet reaches only level of an impurity unavoidably included in theproduction.

According to still another embodiment, the composition of the glasssheet comprises SrO in an content lower than 0.1 wt %. Preferably, thecomposition of the glass sheet is free of SrO. This meant that theelement strontium is not intentionally added in the glass batch/rawmaterials and that, if it is present, SrO content in the composition ofthe glass sheet reaches only level of an impurity unavoidably includedin the production.

According to still another embodiment, the composition of the glasssheet comprises bulk SnO₂ in an content lower than 0.1 wt % (bulkcontent excluding SnO₂ in the “tin face” of a float glass sheet).Preferably, the composition of the glass sheet is free of bulk SnO₂.This meant that the element tin is not intentionally added in the glassbatch/raw materials and that, if it is present, bulk SnO₂ content in thecomposition of the glass sheet reaches only level of an impurityunavoidably included in the production.

According to a preferred embodiment of the invention, the compositioncomprises coloring components other than iron, chromium and cobaltoxides in a total content which less than 0.005 wt %. Such an embodimentallows to control color and thus to provide a glass sheet which isneutral as mainly requested for display applications. More preferably,the composition of the invention comprises coloring components otherthan iron, chromium and cobalt oxides in a total content which is lessthan 0.003 wt %.

Advantageously, the composition of the invention may further comprisechromium and/or cobalt oxides in a total content which is between 0.001and 0.025 wt %. This meant that the composition may comprise onlychromium, only cobalt or both. Such a specific composition makes theglass especially suitable for touch technology based on IR transmission.

According to one embodiment of the invention, the glass sheet is coatedwith at least one transparent and electrically conducting thin layer. Atransparent and conducting thin layer according to the invention can,for example, be a layer based on SnO₂:F, SnO₂:Sb or ITO (indium tinoxide), ZnO:Al or also ZnO:Ga.

According to another advantageous embodiment of the invention, the glasssheet is coated with at least one antireflection layer. This embodimentis obviously advantageous in the case of use of the glass sheet of theinvention as front face of a screen. An antireflection layer accordingto the invention can, for example, be a layer based on porous silicahaving a low refractive index or it can be composed of several layers(stack), in particular a stack of layers of dielectric materialalternating layers having low and high refractive indexes andterminating in a layer having a low refractive index.

According to another embodiment, the glass sheet is coated with at leastone anti-fingerprint layer or has been treated so as to reduce orprevent fingerprints from registering. This embodiment is alsoadvantageous in the case of use of the glass sheet of the invention asfront face of a touchscreen. Such a layer or such a treatment can becombined with a transparent and electrically conducting thin layerdeposited on the opposite face. Such a layer can be combined with anantireflection layer deposited on the same face, the anti-fingerprintlayer being on the outside of the stack and thus covering theantireflection layer.

According to still another embodiment, the glass sheet is coated with atleast one layer or has been treated so as to reduce or prevent glaringand/or sparkling. This embodiment is of course advantageous in the caseof use of the glass sheet of the invention as front face of a displaydevice. Such an anti-glare or anti-sparkling treatment is for example anacid-etching producing a specific roughness of the treated face of theglass sheet.

According to the applications and/or properties desired, otherlayer(s)/treatment(s) can be deposited/done on one and/or the other faceof the glass sheet according to the invention.

The invention also relates to a glass sheet according to the inventionwhich is chemically tempered. All previously described embodiments andpreferred composition ranges also apply to the invention of chemicallytempered glass sheet.

Finally, the invention also relates to the use of the chemicallytempered glass sheet according to the invention in an electronic device.

Embodiments of the invention will now be further described, by way ofexamples only, together with some comparative examples, not inaccordance with the invention. The following examples are provided forillustrative purposes, and are not intended to limit the scope of thisinvention.

EXAMPLES

Powder raw materials were mixed together and placed in meltingcrucibles, according the compositions specified in the following table.The raw material mix was then heated up in an electrical furnace to atemperature allowing complete melting of the raw material.

Comparative Comparative Wt % ex. 1 (SL) ex. 2 (AS) Ex 1 Ex2 Ex3 Ex4 Ex5Ex6 Ex7 SiO₂ 71.6 60.9 70.5 72.7 73.0 74.0 73.9 73.1 73.1 Al₂O₃ 1.1 12.83.1 1.2 1.1 1.1 1.2 1.1 1.1 MgO 8.1 0.1 3.4 3.3 3.4 0.3 0.4 3.5 3.6 CaO4.3 6.7 7.2 7.2 7.1 8.9 8.9 6.9 6.9 Na₂O 13.7 12.2 13.8 13.7 14.8 15.315.3 14.9 14.9 K₂O 0.0 5.9 1.7 1.7 0.2 0.2 0.2 0.2 0.2 Fe₂O₃ 0.09 0.020.09 0.09 0.09 0.09 1.10 0.09 1.10 BaO 0 0.2 0 0 0 0 0 0 0 SO₃ 0.36 00.36 0.36 0.36 0.36 0.36 0.36 0.36 SrO 0 0.2 0 0 0 0 0 0 0 ZrO₂ 0 1.0 00 0 0 0 0 0

After the melting and the homogenization of the composition, the glasswas cast in several small samples of 40*40mm and annealed in anannealing furnace. Subsequently, the samples were polished up to asurface state similar to floated glass (mirror polishing). Severalsamples were produced for each composition. Composition of comparativeexample 1 corresponds to a classical soda-lime (SL) glass according tothe state of the art and composition of comparative example 2corresponds to a commercially available alumino-silicate (AS) glass.Compositions of example 1-7 correspond to compositions according to theinvention, combining low alumina and low calcium oxide content, and a“reverse” (CaO/MgO) ratio.

Chemical Tempering

The samples prepared in above section were chemically tempered at thesame time and in the same conditions. The samples of differentcompositions were placed in a cassette, preheated and then dipped in amolten KNO₃ (>99%) bath at 420° C. for 220 minutes. After the ionexchange, the samples were cooled down and washed. Subsequently thesurface compressive stress (CS) and the depth of exchanged layer (DoL)were measured via photoelasticimetry. The following table summarize theaverage value of CS and DoL for ten random samples of each ofcompositions 1-7 according to the invention and compositions ofComparative examples 1-2.

Comparative Comparative ex. 1 (SL) ex. 2 (AS) Ex1 Ex2 Ex3 Ex4 Ex5 Ex6Ex7 Surface 791 884 808 695 743 645 646 705 693 compressive stress (Mpa)Depth of 6.3 36.1 13.9 14.3 13.0 20.3 18.6 13.5 12.6 exchanged layer(μm)

Those results show that combining a low content in Al₂O₃ and CaO with areverse (CaO/MgO) ratio gives low cost and easily produciblecompositions, allowing to significantly improve the depth of exchangedlayer, while keeping a high surface compressive stress and thus, toincrease the glass reinforcement.

Moreover, DOL values of compositions according to the invention are wellappropriate for a “piece-by-piece” process used to produce cover glassfor display devices (preferably higher than 10 microns and verypreferably higher than 12 microns or even better higher than 15microns).

Other Properties

The following properties were evaluated on the basis of glasscomposition using Fluegel model (Glass Technol.: Europ. J. Glass Sci.Technol. A 48 (1): 13-30 (2007); and Journal of the American CeramicSociety 90 (8): 2622 (2007))—for compositions of examples 1-4 accordingto the invention as well as of Comparative examples 1-2:

-   -   Glass melt density evaluated at 1200 and 1400° C.;    -   Viscosity through the “Melting point temperature T2”;    -   “Working point temperature T4”;    -   Devitrification temperature T0;    -   Coefficient of thermal expansion (CET);

In a general manner:

The melting point temperature T2 is preferably at most 1550° C., morepreferably at most 1520° C., the most preferably at most 1500° C.

The Working point temperature T4 is preferably at most 1130° C., morepreferably at most 1100° C., the most preferably at most 1070° C.

The devitrification temperature T0 is preferably at most T4, morepreferably at most T4−20° C., the most preferably at most T4−40° C.

CET value is preferably at most 9.6 and more preferably at most 9.4.

Comp. Comp. ex. 1 ex. 2 (SL) (AS) Ex 1 Ex2 Ex3 Ex4 Ex5 Ex6 Ex7 Glassmelt 2.37 2.32 2.35 2.34 2.34 2.33 2.33 2.34 2.34 density (1200° C.)Glass melt 2.33 2.32 2.33 2.32 2.32 2.31 2.31 2.32 2.32 density (1400°C.) Melting point 1459 1601 1499 1487 1478 1485 1487 1475 1478 T2 (° C.)Working point 1032 1176 1058 1049 1045 1047 1050 1040 1044 T4 (° C.)Devitrification 993 951 986 983 948 989 990 930 932 temperature T0 (°C.) CET @210° C. 9.11 9.68 9.23 9.17 9.08 8.90 8.93 9.08 9.10 (10⁻⁶/K)

The compositions according to present invention are suitable for formingby a float process and while using existing furnace tools for productionof soda lime glass because of:

-   -   their melting point temperature T2 being lower than 1500° C. and        which are comparable to a classical soda lime glass (Comparative        ex. 1) and significantly lower compared to an aluminosilicate        glass (Comparative ex. 2);    -   their working point temperature T4 which is lower than 1100° C.        and which are comparable to a classical soda lime glass        (Comparative ex. 1) and lower compared to an aluminosilicate        glass (Comparative ex. 2);    -   their devitrification temperature T0 are suitable because lower        than working point temperature T4;    -   their glass density which is very close to soda lime and        aluminosilicate glasses (Comparative ex. 1-2), thereby        avoiding/limiting density defects during composition change        (transition);

Moreover, the compositions according to present invention havecoefficients of thermal expansion (CET) which reach in a known mannerappropriate values for a subsequent chemical tempering (limitingdifferentiated cooling deformation phenomenon). More specifically, thecompositions according to present invention show better (lower) valuesfor CET than aluminosilicate glass and thus are less sensitive todifferentiated cooling issues than AS glass.

Finally, compositions according to the invention allow to get sulfatefining ability during their manufacture/melting, thanks to an adequatesolubility of sulfate and suitable high-temperature viscosity.

1. A glass sheet having a boron- and lithium-free glass compositioncomprising the following in weight percentage, expressed with respect tothe total weight of glass: 65≦SiO₂≦78% 5≦Na₂O≦20% 0≦K₂O<5% 1≦Al₂O₃<4%0≦CaO<4.5% 4≦MgO≦12%; as well as a (CaO/MgO) ratio which is less than 1.2. The glass sheet according to claim 1, wherein the compositioncomprises total iron (expressed in the form of Fe₂O3) in a contentranging from 0.002 to 1.7% by weight.
 3. The glass sheet according toclaim 1, wherein the composition comprises total iron (expressed in theform of Fe₂O₃) in a content ranging from 0.002 to 0.6% by weight.
 4. Theglass sheet according to claim 1, wherein the composition comprisestotal iron (expressed in the form of Fe2O3) in a content ranging from0.002 to 0.2% by weight.
 5. The glass sheet according to claim 1,wherein the composition comprises total iron as follows: 0.06<Fe₂O₃≦1.7%by weight.
 6. The glass sheet according to claim 1, wherein thecomposition comprises total iron as follows: 0.06<Fe₂O₃≦0.6% by weight.7. The glass sheet according to claim 1, wherein the compositioncomprises total iron as follows: 0.06<Fe₂O₃≦0.2% by weight.
 8. The glasssheet according to claim 1, wherein the composition comprises: 1≦Al₂O₃≦3wt %.
 9. The glass sheet according to claim 1, wherein the compositioncomprises: 2≦Al₂O₃≦3 wt %.
 10. The glass sheet according to claim 1,wherein the composition comprises: 0<CaO<4 wt %.
 11. The glass sheetaccording to claim 1, wherein the composition comprises a (CaO/MgO)ratio which is less than or equal to 0.45.
 12. The glass sheet accordingto claim 1, wherein the composition comprises a (CaO/MgO) ratio which isless than or equal to 0.4.
 13. The glass sheet according to claim 1,wherein the composition comprises a (CaO/MgO) ratio which is less thanor equal to 0.1.
 14. The glass sheet according to claim 1, which ischemically tempered.
 15. Use of the a glass sheet according to claim 1in an electronic device.